In general, a substrate of a III-V compound semiconductor crystal, in particular, a compound semiconductor single crystal of, for example, GaAs, InP, or GaP, is subjected to a process of mirror-polishing at least one major surface of the substrate with a chlorine-based polishing agent, subsequently cleaned with an acid or an alkali, and finally rinsed with ultrapure water and dried.
For example, Patent Literature 1 describes a cleaning method in which a natural oxide layer is formed in a surface of a GaAs substrate that is being polished, and the natural oxide layer is dissolved and removed from the surface of the GaAs substrate with ultrapure water having a dissolved oxygen content of 1 ppm or less.
Patent Literature 2 states that the surfaces of a substrate are oxidized and the substrate is then immersed in aqueous ammonia, an aqueous solution of sodium hydroxide, phosphoric acid, hydrochloric acid, or hydrofluoric acid.
Patent Literature 3 describes a cleaning method in which organic matter and metal on a surface of a substrate are removed and an oxide film of the substrate is then etched with an acidic solution; the substrate is then subjected to cleaning with an alkaline aqueous solution, subsequently to cleaning with ultrapure water, and then to drying. According to this method, foreign matter such as deposits can be completely removed.